Sig does not make/sell a barrel to convert the P229 .40 S&W to 9mm. If you want to stick to Sig parts, you'll have to get the slide, barrel, and spring; and Sig does not sell them that way.

To convert your .40 P229 to 9mm, BarSto (and maybe others) make a conversion barrel that may or may not drop right in. I believe it is usually referred to semi-drop in, meaning it may require some minor gunsmith fitting.

Sig's .40 and .357 barrels are interchangeable in the P229 because they both have the same external diameter at the muzzle. The external diameter of the Sig 9mm barrel is smaller, as is the hole in the muzzle of the slide. The .40/.357 slide is different from the 9mm slide both in the dimension of the muzzle hole and some other places.